I was playing with a dude on Standoff Domination, he got a VSAT, followed by Lodestar, his score was something like 50-2 by now and then he called in his Swarm and got killed by his own Swarm lol, then he ragequited with 68-3. Who the hell ragequits with a 68-3 score LOL.
Or, they were boosting/ cheating and got booted mid game. I have played games on previous cods where someone was cheating, I filed a complaint, within seconds they were gone. No one voluntarily leaves a game when they're numbers are like that. Remember, the banhammer has been at work.
They don't always. It depends on the host and the other players connections in the game. No matter what, it's always best to be the host since the host see's the least amount of lag (or can chose to dash).